Ballistic Helmets
The current U.S. Army Advanced Combat Helmet (ACH) is an improvement over the PASGT system (Personnel Armor System-Ground Troops). The PASGT helmet was developed in the late 1970's by research scientists at the U.S. Army Natick Research, Development and Engineering Center, Natick, Mass.
The PASGT helmet with an areal density of approximately 330 oz/yd2 (2.30 psf or 11.2 Kg/m2) is still being manufactured using a Kevlar® fabric coated or impregnated with a thermoset resin that is a 50/50 blend of the phenol thermoset and a polyvinyl butyral thermoplastic toughener, which is referred to as a “prepreg”. The PASGT helmet fabric is made with a 1500 denier Kevlar® 29 yarn in a 2×2 basket fabric construction that weighs 14.0 oz/yd2 (475 g/m2). The Kevlar® prepreg is a Kevlar® fabric impregnated with 16-18% by weight of Polyvinyl Butyral (PVB)-phenolic resin. The helmets are fabricated by assembling a helmet preform using 19 equivalent layers of prepreg. These layers are then compression molded at constant temperature and rather substantial compression pressures using self-trimming matched metal molds at a rate of one helmet every 12-15 minutes. (1 oz/yd2=33.94 gr/m2).
Because of the thermoset nature of the PVB-phenolic resin matrix, molding of the standard PASGT helmets requires constant temperatures between 320-355° F. (160-180° C.) and pressures well over 500 psi for roughly 12-15 minutes on average for the resin to fully cure.
The ballistic properties of the helmet are normally measured by the ballistic limit V50 (m/s) test, according to known US and European standards. In general, a ballistic limit V50 (f/s) is measured to determine the ballistic performance of a material or system and is defined as the velocity at which the probability of penetration of a bullet or projectile is 50 percent. The test procedure is specified on the MIL-STD-662F latest revision dated Dec. 18, 1997. Although the military MIL-H44099-A specification for PASGT helmets requires a ballistic performance with a ballistic limit, V50 (f/s), of at least 2000 f/s, most PASGT helmets measure around 2100 f/s V50 against 17 grain fragment simulators projectiles (FSP) during quality assurance lot testing.
In the case of a medium size PASGT helmet, 19 layers were required to deliver a helmet with the target weight or areal density of 2.3 lb/ft2 (11.2 Kg/m2). To obtain 19 equivalent layers during the preform assembly, the helmet preform was put together using 16 pinwheels/rosettes and 3 crown pieces. The reason is because the pinwheels/rosettes petals (legs) overlap during the preform assembly, providing an increase of layers on the side of the helmet walls higher than the crown area. Therefore, crown patches of preform material are placed in the crown area of the helmet to compensate and balance the number of layers.
In addition, using Kevlar® fabrics impregnated with PVB-phenolic resins (prepreg) as preform material, “directionality” of the fibers within the fabric has never been a consideration to provide higher ballistic performance in a helmet. In fact, when the KM2 Kevlar® helmet was developed at DuPont back in 1990, the ballistic improvements came from improvements in the properties of the fibers, fabric and surface treatment. Thus, the KM2 helmet produced a 15% lighter helmet shell with superior ballistic performance to the incumbent Kevlar® 29 PASGT helmet system by using 850 denier Kevlar® KM2 fiber, a higher toughness Kevlar® fiber than the 1500 denier Kevlar® 29 used in the standard PASGT. The helmet also used a finer Kevlar® yarn denier, a greater ply count with a lighter fabric and a special fiber surface treatment to control the adhesion strength between the PVB-phenolic resin and the fiber surface.
DuPont presented the concept of improved ballistic performance of a helmet by using a greater number of Kevlar® prepreg layers (ply count) with lighter fabrics in a paper dated Oct. 22, 1991, at the Society for the Advancement of Materials and Process Engineering (SAMPE) International Technical Conference (see, Riewald, P. G., F. Folgar, H. H. Yang, W. F. Shaughnessy, “Light Weight Helmet from a New Aramid Fiber,” 23rd International SAMPE Technical Conference, Oct. 21-24, 1991, Kiamesha Lake, N.Y. Vol. 23, 684-695).
One of the US Army greatest challenges has been to reduce the weight of personnel body armor carried by a soldier, including ballistic vests and helmets. Recent studies have identified material combinations that could meet both structural and ballistic requirements at lighter weights. More compliant matrix ballistic materials suggest the potential for greater ballistic efficiencies than the existing helmet materials. High ballistic efficiency fibers such as p-aramid, PBZ and ultra-high molecular weight polyethylene (UHMWPE) when combined with a thermoplastic (TP) matrix will improve the ballistic protection beyond that afforded by the incumbent phenolic thermoset resin systems.
High Performance Fibers, Fabric Structures and Matrix Resins
Some of the limitations for further ballistic improvements in a helmet when using a finer Kevlar® yarn denier, a greater ply count with a lighter fabric and a special fiber surface treatment come from the use of woven fabrics itself. As shown in FIG. 1, during a ballistic impact, the individual yarns in a woven fabrics are constrained at the crossover points 110. The tensile wave propagated along the longitudinal direction of the yarns is reflected at the crossover points breaking the yarns much before the maximum amount of energy can be absorbed along its length. Therefore one way to improve the ballistic performance of woven fabrics was to reduce the cross over points by reducing the fabric crimp and/or spreading the crossover points farther apart by weaving fabrics in a more open construction.
These open fabric constructions resulted, for the same yarn denier, in more layers (ply count) of a lighter fabric to meet given ballistic requirements. The physics behind the improvement in ballistic performance due to more layers of a lighter fabric for the same total areal density is attributed to a more uniform distribution of the impact energy throughout more fabric layers and a reduced density of crossover points.
Since there is a limit to how open a woven fabrics could be before it loses all its properties as a ballistic fabric due to becoming like an open net, the next step to improve the ballistic performance of woven fabrics was to reduce the yarn crimp. Modern weaving equipment spread the filaments of the yarns in a flat and oriented fashion on a loom giving rise to what is currently known as uni-directional fabric construction. Since uni-directional fabric constructions have very little crimp, two of the most common methods to hold together two or more layers of uni-directional fabrics are by using a low modulus thermoplastic film to bond the fabric layers together or using fine denier yarns of low tensile modulus and low strength fibers to stitch the layers together.
A common uni-directional fabric construction has two layers with their yarns at 90 degrees from each other bonded together by a thermoplastic film or stitched together by a fine denier yarn (FIG. 2). This uni-directional construction is known as 0°/90° fabric, or 0/90 for short. When a uni-directional fabric construction requires more than two layers, the orientation of the yarns change only 90 degrees from the yarn orientation of the next layer. As an example, in a four layers uni-directional fabric construction, the orientation of the yarns will have a 0°/90°/0°/90° orientation sequence.
When compared with woven fabrics, the yarns are less constrained in the uni-directional fabric constructions because the crossover points and the crimp have been highly reduced resulting in ballistic articles with a much lower areal density for the same ballistic requirements. The areal density of an article is its weight divided by its area. The higher ballistic efficiency of uni-directional fabrics is then attributed to the fact that the yarns can dissipate more impact energy along their length resulting in the impact energy being transmitted to a much larger area within each uni-directional fabric layer away from the place of impact.
Some examples of commercially available uni-directional fabric constructions made with para-Aramid fiber yarns, hereby incorporated by reference, are disclosed on patents issued to Andrew Park and a patent issued to Barrday. Andre Park's patents (U.S. Pat. Nos. 5,635,288; 7,148,162; 5,952,078) discloses a uni-directional fabric construction of two layers of high performance fibers cross-plied (0/90 orientation) bonded together by a thermoplastic film or scrim where it claims that the film or scrim, once laminated by heat and pressure, do not penetrate the fiber filaments inside the yarn. Barrday's patent US 2007/0099526 also discloses a two layers 0/90 uni-directional fabric construction where it claims that the yarns are held together by stitching with a finer lower modulus yarn that Barrday refer as encapsulating yarns. In both cases, when testing articles with multiple layers using the same fiber yarn denier and with the same total areal density, they have demonstrated that the uni-directional fabric constructions have higher ballistic performance than the equivalent woven fabrics.
A more relevant class of uni-directional materials for fabricating high performance ballistic helmets are commercially available with the trade names of Spectra Shield from Honeywell International and Dyneema from DSM-Dyneema Corporation. Both of these brands are sheet-like array of high performance Ultra High Molecular Weight Polyethylene (UHMWPE) fiber filaments in which the filaments are aligned parallel to each other and coated or impregnated with a thermoplastic elastomer matrix as a prepreg. Hereby incorporated by reference, Honeywell patent U.S. Pat. No. 4,916,000 discloses the basic uni-directional sheet structure of Spectra Shield where it claims that the matrix impregnates the single filaments. U.S. Pat. No. 5,173,138 patent teaches a method for the continuous and automatic production of the cross-plied Spectra Shield material where two layers of the sheet-like prepreg are cross-plied together at a 90 degree angle to produce a single sheet 0/90 construction of ballistic material.
Helmet Preform
As discussed above, the PASGT helmets are fabricated by assembling a helmet preform using 19 equivalent layers of prepreg. These layers are then compression molded at constant temperature under rather substantial compression pressures using self-trimming matched metal molds at a rate of one helmet every 12-15 minutes. In the case of a medium size PASGT helmet, 19 layers were required to deliver a helmet with the target weight or areal density of 2.3 psf (lb/ft2). To obtain 19 equivalent layers during the preform assembly, the helmet preform was put together using 16 pinwheels/rosettes 310 and 3 crowns 320 (FIGS. 3a and 3b). The reason is because the pinwheels/rosettes petals (legs) 412 overlap during the preform assembly (FIG. 4), providing an increase of layers on the side of the helmet walls higher than the crown area. Therefore, the crown patches 320 of preform material are placed in the crown area of the helmet to compensate and balance the number of layers, FIG. 3 (b).
The use of preform patterns to facilitate the fabrication of helmets is mentioned as early as 1942 in US patents including, for example, U.S. Pat. Nos. 2,420,522, 2,423,076, 2,532,442 and 2,610,322 to Le Grand Daly, as well as U.S. Pat. No. 2,451,483 to Goldsmith. Daly's patents, hereby incorporated by reference, teach the use of the pinwheel helmet preform pattern. In particular, U.S. Pat. Nos. 2,423,076 and 2,532,442 are directed to a conventional practice of making safety helmets from flat sheets of plastic impregnated fabrics in a speedier and more convenient manner. Daly patents uses the shape of the individual pieces, already referred to as “preforms” or “pre-forms”, and their arrangement in the helmet in a way that the seams are mutually staggered, assembled as going into the molding dies without bending or moving any pieces from their respective places in the “preforms,” preventing formation of wrinkles and folds, and yet no thick portions having excess of material in the helmet. FIG. 5 is a view illustrating the “preforms” 30,31 from Daly prior to and after being folded together to form a bowl-shaped assembly 510, which is ready to be assembled for molding.
Forty years later in the early 1980's, with the availability of Kevlar® as a new high performance fiber developed and commercialized by E.I. DuPont de Nemours, and better suited for the fabrication of ballistic helmets; it was apparent that materials cost became a pressing issue since a large number of plies were required for a specific level of ballistic protection. It was also apparent that although using Daly's preform patterns to be laid up in a mold and pressed into a helmet resulted in a high quality method of making a helmet; it did not make efficient use of the prepreg material because of the waste during the cutting process.
In 1983 Gentex Corporation filed for a patent addressing a more efficient use of prepreg materials by introducing a preform pattern design which substantially reduced the amount of prepreg needed to make a helmet shell, see, for example, Grick's U.S. Pat. No. 4,596,056. As shown in FIG. 6a, a helmet pinwheel pattern 610 increased the efficiency of the prepreg materials by the method used in putting together the pinwheel. FIG. 6b shows a single pinwheel segment 620 that is joined together with seven other pinwheel segments 620 by stitching or by a heat sensitive tape at the center to form the pinwheel pattern 610 of FIG. 6a. Cutting pinwheel segments 620 with the same shape and size reduced the waste during the cutting process from a roll of fabric prepreg.
Since the assembly of a preform into a molded helmet shell leaves one ballistically weak spot on top of the helmet, Gentex Corporation later improved their helmet preform pattern design as in U.S. Pat. Nos. 4,778,638 and 4,908,877 in which a ballistic helmet is made with an efficient use of the prepreg fabric material and without sacrificing any degree of ballistic protection around the whole helmet. This is accomplished by using preform patterns of hexagonal shape 710 that have a slit from the corners of the hexagon up to a certain distance from the center as shown in FIG. 7. The efficiency of the prepreg fabric usage came from the reduction of waste during cutting the hexagonal shape of the preform patterns and the staggering of the hexagonal patterns of different sizes during the assembly of the preform before being laid into the compression molding dies.
Molding of Ballistic Helmets.
In general, some of the molding methods known in the prior art for making ballistic helmets includes the use of matched metal dies, i.e., a male mold 810 and a female 820 mold for the compression molding of a preform 830. As an example, because of the thermoset nature of the PVB-phenolic resin matrix, compression molding of a Kevlar/PVB-phenolic PASGT helmet 840 requires constant temperatures between 320-355° F. (160-180° C.) and pressures over 500 psi for about 12-15 minutes on average for the resin to fully cure, as shown in FIG. 8.
Other molding methods for making helmets include the use of a metal female mold and a flexible bladder to form the helmet preform, consolidate it, and cure it under high temperature and pressure as disclosed in U.S. Pat. Nos. 2,420,522, 2,423,076, 2,532,442 and 2,451,483.
U.S. Pat. Nos. 2,420,522, 2,423,076 and 2,532,442 are directed to a molding method for making safety helmets using plastic impregnated fabrics. The method includes the steps of folding the pinwheel patterns into a preform closer to the helmet shape, assembling the preform and laying down the helmet preform inside a female cavity. The method also includes using pinwheel patterns to reduce or eliminate wrinkles or folds when the pinwheel edges (petals) overlap, therefore avoiding the breaking or damaging of the fibers in the fabric during the preform assembly step. The final steps of of the method include molding and curing of the helmet preform shown in FIG. 9. In FIG. 9, which corresponds to FIG. 18 from U.S. Pat. No. 2,420,522, the helmet preform is seen laying down on the surface of a female metal mold than is later pressed against the wall of that mold by the expansion of a rubber bladder under the action of hydraulic pressure. In Daly's molding method, a uniform pressure is exerted at all points and in all direction on the helmet while being molded and cured. The molding pressures used were much lower than 1000 pounds per square inch (psi) and applied during the entire curing cycle of the helmet. This was particularly important to the invention because it was considered that the fibers in a plastic impregnated fabric were damaged if the prepreg was stretched during the molding process as in the stretching of a metal sheet during stamping. Daly's patents also showed the use of the rubber bladder and a female cavity for molding and curing a helmet, the need for steel match-metal dies was eliminated, which were more expensive and took a long time to fabricate.
U.S. Pat. No. 2,451,483 disclosing a similar molding method as Daly for making protective helmets where the use of clover leaf shaped pinwheels and a female mold cavity capable of incorporating a part line, drill hole marks and a negative surface draft were also disclosed.
U.S. Pat. No. 4,338,070 to Pier L. Nava from Italy discloses a molding equipment and process to form a complex shape such a helmet inside a female cavity by hydraulically pressing a flexible bladder as a male mold. Navas also claims a better liquid infiltration molding process than an autoclave for resin to penetrate a fabric preform in order to obtain uniform wall thickness of complex parts containing negative surface draft and surface projections is shown in FIG. 10. FIG. 10, which is hereby incorporated in its entirety by reference, corresponds to FIG. 3 from Nava.
Dickson from Armorsource filed International Patent Application Publication Number WO2010019697 that was directed to the use of a flexible elastomeric bladder to pressurize a helmet preform by hydraulic means placed in between a female cavity and a male core as shown in FIG. 11. FIG. 11 corresponds to FIG. 3 in WO2010019697 with striking similarities to FIG. 3 in U.S. Pat. No. 4,338,070. Dickson is also directed to the use of uni-directional prepreg materials and the need for very high pressures to be continuously and uniformly applied during all the stages of molding and curing of the helmet preform. Dickson also disclosed that both elevated temperatures and pressures are required to be maintained throughout their entire molding process where the resin from the prepreg material melts and undergoes a phase change in order to form a composite helmet. The patent also teaches the use of helmet prepreg pinwheel patterns (packets) that can be formed into a helmet shape preform in a pre-molding step before molding to facilitate the assembling of a three-dimensional shape without additional wrinkles and folds.
Helmet Ballistic Performance
Since the late 1980s thru the 1990s improvements with ballistic performance and weight reduction of PVB-phenolic aramid fabric helmet systems included the 850 denier Kevlar® KM2 fiber system developed by DuPont in 1990. Table 1 provides a comparison between the PASGT helmet and the KM2 helmet.
TABLE 1Ballistic Performance of PASGT vs. KM2 Kevlar Helmets.PropertiesPASGT HelmetKevlar ® KM2 HelmetAreal Density, psf2.301.95Yarn Denier/Fiber1500 denier,850 denier,Kevlar ® 29Kevlar ® KM2Fabric2 × 2 basket,31 × 31 plain weave,Construction/Weight475 g/m2234 g/m2Matrix Resin/WeightPVB-phenolic,PVB-phenolic,Fraction16-20%16-18%Molding ProcessCompressionCompressionMoldingMoldingMolding Cycle time, min10-15min10-15minV50 (f/s), 17 grain fsp2,100f/s2,200f/s
As seen in Table 1, the KM2 helmet is a 15% lighter helmet shell with superior ballistic performance to the Kevlar® 29 PASGT helmet system. This improvement was achieved mostly by using a higher toughness Kevlar® fiber than the 1500 denier Kevlar® 29 used in the standard PASGT, finer Kevlar® yarn denier, greater ply count and a special fiber surface treatment to control the adhesion strength between the PVB-phenolic resin and the fiber surface. However the ballistic performance improvement were between 5-12% range against RCC and chisel nose fragment simulator projectiles (FSP).
More compliant matrix ballistic materials developed during the 1980s and 1990s have shown the potential for greater ballistic efficiencies than the existing helmet para-Aramid/PVB-phenolic materials. High ballistic efficiency fibers such as ultra-high molecular weight polyethylene (UHMWPE), when combined with thermoplastic matrices, improve the ballistic protection beyond that afforded by the incumbent phenolic thermoset resin systems.
Publically available information on ballistic performance of ballistic helmets is limited at best. Most of the available information is for ballistic performance of materials tested as a flat panels. For example, U.S. Pat. No. 6,183,834 to Van der Loo is directed to a ballistic-resistant flat panel made by a compressed stack of layers containing 0/90 uni-directionally oriented reinforcing UHMWPE fibers with up to 30 wt. % of a thermoplastic matrix material to provide protection against impacts of projectiles such as shrapnel or bullets. In addition to the Ballistic Limit, V50 (m/s), Van der Loo uses the Specific Energy Absorption (SEA) to measure the amount of energy absorbed by a molded panel on impact of a projectile per unit areal density of the molded panel. SEA is defined as:SEA=½*m*(V50)2/AD,
where m is the mass of the projectile, V50 (m/s) is the ballistic limit of the projectile, and AD is the areal density of the molded panel. Van der Loo found that very high values of SEA of at least 110 Jm2/kg against a 7.62×39 Mild Steel Core P.S. Ball M1943 are achieved when the compressed stack of uni-directional UHMWPE material reaches at least 98.0% of the theoretical maximum density, and most preferably at least 99.5%, by compressing the stack at a pressure of at least 15 MPa (2,175 psi) during heating at an elevated temperature and during cooling down to room temperature. Van der Loo is also directed to molding at an elevated temperature by compressing the UHMWPE material at a temperature above the softening or melting point of the thermoplastic matrix material and below the softening or melting point of the fibers, recommending that the required compression time and compression temperature depend on the kind of fiber, matrix resin and on the thickness of the molded panel. Suggesting that the compression temperature for UHMWPE fibers is preferably between 115 to 130° C. and cooling to below 70° C.
US Patent Publication Number 2007/0194490 to Bhatnagar from Honeywell International, Inc. demonstrated that the ballistic performance and structural characteristics of uni-directional composite materials with thermoplastic matrix increase with increasing the molding pressure. Specifically, the uni-directional para-Aramid fibers (Twaron® T2000 from Teijin) were coated with 16% by weight of a polyurethane resin to make a uni-tape. The uni-tapes were cross-plied at 90° and multi-layer composite flat panels were pressed under different molding pressures.
In Bhatnagar, multiple layers of a 2-ply para-Aramid fiber construction were molded separately at 240° F. (115.6° C.) at a molding pressures of 500 psi (3.4 MPA) and 2500 psi (10.3 MPa) for a period of 20 minutes and allowed to cool to room temperature. Ballistic testing using a 7.62×51 mm M80 NATO bullet was carried out in accordance with NIJ Standard NIJ 0101.04 and demonstrated that the ballistic resistance was substantially higher when molding at 10.3 MPa (2500 psi) of pressure than using the same matrix resin but molding at a low pressure. Bahtanagar also disclosed that a fewer number of layers of the composite formed in accordance with the patent can obtain similar ballistic properties than with a larger number of layers which were molded under lower pressure.
However, Bahtanagar also shows that the increase in ballistic performance and structural characteristics of the uni-directional para-Aramid composite material is not guaranteed by increasing the molding pressure alone if the resin matrix is changed. Bahtanagar illustrated this behavior of uni-directional composite materials by using two different matrix resins, one thermoplastic and one thermoset. The thermoplastic was Kraton® D1107 styrene-isoprenestyrene block copolymer thermoplastic elastomer at 20% by weight and molded at 250° F. (121.1° C.) for 30 minutes. The thermoset resin was an epoxy vinylester resin (Derkane 411) at also 20% by weight and molded at 200° F. (93.3° C.) for 30 minutes. In both cases, the ballistic improvement by increasing the molding pressure was not observed. In addition, ballistic improvements of those uni-directional para-Aramid composite materials with either Kraton® or epoxy vinylester resins were not observed when testing against a 9 mm FMJ hand gun bullet either.